PTP is used to synchronize a master clock with slave clocks where extreme precision is required, such as in advanced control or cellular communications systems. Where there are a number of potential master clocks, a best master clock algorithm may select the best clock to use as the master based on predetermined requirements. The selected clock is known as the grand master.
As described in the IEEE standard referenced above, PTP makes use of timestamped synchronization packets to carry timing information over the network to the receiver, where a physical clock signal is recovered using a phase locked loop driving a digitally controlled oscillator. An exemplary prior art PTP clock recovery system is described in U.S. Pat. No. 7,689,854, the contents of which are herein incorporated by reference. Further details are provided in the Hirschmann White paper entitled Precision Clock Synchronization—IEEE 1588, Rev. 1.2, the contents of which are herein incorporated by reference.
Clock recovery often takes place at the boundary between networks. The requirements for boundary clocks are described by ITU recommendation G.8273.2, the contents of which are herein incorporated by reference. Prior art clock recovery devices may not be able to meet the extremely strict requirements with regard to the ability to reduce residual phase error when used as boundary clocks.
Prior art clock recovery devices are based on using the same input phase sample for both frequency and phase paths of the closed loop PLL. In addition, the steady state phase and frequency locked loop incorporated in such devices assumes that the fast phase locking mechanism of the non-linear phase adjustment block corrects the phase every nano-second of the first phase offset seen by the system. This has proved problematic especially during mode and reference switching where this approach causes a permanent residual phase error that the clock recovery device is unable to correct.